Sampling circuit fault is one of the important reasons for causing rotor ground fault protection misoperation. To solve this problem， a method by double sampling circuit and two algorithms to judge the rotor ground fault protection is proposed. This method uses different sampling data and two algorithms to calculate the rotor ground resistor values separately. When all of the two calculated resistor values are less than the setting value of the one-point ground fault protection， it is judged that one-point ground fault for generator rotor is occurred. By this method， the reliability of the one-point ground fault protection is increased， and the rotor ground fault protection misoperation caused by sample circuit fault can be effectively prevented.